Separate pattern by Z coordinates
The arctan2 into division step is turning this into a radial gradient.  Instead of a light and a dark half you have light and dark quarters of the mesh, basically, and the way you've ramped the color is kind of hiding the problem from you.

So I think what you are asking for is not going to happen without some changes to the math leading up to it.  If you take the output of your noise texture into a math node set to Less Than you may find something you like, or try the following: